The present invention relates to a process and transducer for emitting wide band and low frequency acoustic waves in unlimited immersion depth.
The technical field of the invention is the fabrication of electro-acoustic transducers for the emission of acoustic waves in a fluid.
The main application of the invention is the possibility of emitting low frequency acoustic waves, at great depth and in a wide enough frequency band.
Immergeable electro-acoustic transducers are known, especially piezoelectric ones. The present invention is preferably intended for them although not limited to them. The transducers comprise a rigid hollow cylindrical shell, open at both axial ends, and inside which two identical electro-acoustic motors are arranged coaxial with the latter. The transducers are located on both sides of a central countermass and have opposite ends surrounded by a pavilion. The electro-acoustic motors may consist of two stacks of aligned piezo-electric wafers. The external faces of both pavilions are located in the plane of the shell axial ends, so that they are in contact with the fluid in which the shell is plunged, and the external perimeter of these pavilions is as close as possible to the edge of the open axial ends of the shell.
The external faces emit acoustic waves in the fluid when the electro-acoustic motors are electronically energised. These transducers are especially used to emit low frequency acoustic waves in water in a specified direction.
However, one problem formulated by this type of transducer is the propagation of acoustic waves emitted by the pavilion rear faces inside the shell if the latter is also full of fluid and which are then retransmitted into the ambient medium despite the rigidity of the shell, and interfering with the transducer global emission, as indicated in the second solution hereafter described for the transducers immerged at a great depth.
Various solutions have indeed been envisaged and proposed by manufacturers and/or users, such as for example the use of watertight shells filled with gas. However, the shell is required to resist the pressure of immersion in the fluid, and as a result the weight of the transducer is considerably large when the immersion depth is very significant.
Another solution consists in placing masses or static dampers, such as foam, at the pavilion rear part surrounding the end of the electro-acoustic motors, which then absorb the rear radiation and constitute what are called "baffles" with the pavilions. The application of this solution is also limited in deep immersion, since the masses or dampers must be able to resist the pressure, unless this solution is combined with the previous one, i.e., with a rigid shell, which increases the system weight.
In fact, both previous solutions are only extrapolations of the solutions accepted for the wave emission in air.
For normal and very significant immersion depths, four other types of categories of solutions have been developed and various patents have been filed.
A first category of solutions consists of compensating for the external pressure by increasing the internal pressure in different ways in order that a watertight shell does not have to withstand the efforts of resistance to the external pressure:
A particular patent application No. FR 2 634 292 by Mr. Gilles Grosso, relates to a "process and device to maintain the gas contained in an immerged enclosure in pressure balance with the outside", filed on Jul. 15, 1988 is to be noticed. The process consists in associating several bottles, each containing a pre-inflated ductile pocket, at various pressures, to the immersed enclosure such as the shell of a piezo-electric transducer, thus making it possible to compensate the hydrostatic pressure at various immersion depths. PA1 An example of a known patent application FR 2 665 814 of Aug. 10, 1990 filed by THOMSON company relates to "electro-acoustic transducers intended to be immersed", and comprises a system of automatic compensation of the immersion pressure by means of chambers filled with gas and having reduced volumes, in order to compensate only for the axial efforts exercised on the central ceramic pillar of the transducer. PA1 an opening is made in the shell causing the cavity to communicate with the ambient medium; PA1 at least one flexible bladder is installed in all or part of the volume of the cavity; PA1 this bladder is filled with a fluid more compressible than the fluid. PA1 1,725 kg/m.sup.3, and the sound propagation speed C in such a fluid is 570 meter/second, which corresponds to a compressibility defined by the product:
Other patent applications use pneumatic systems to compensate for the external immersion pressure. However, these devices comprise all the mechanical and/or gas supply or storage devices, which are either voluminous and/or complicated. Further, these various devices, avoid the rear wave propagation but do not make it possible to emit on a wide enough frequency range in low frequency, since the wave emission by the pavilions only has a very narrow frequency spectrum, passing through a maximum peak which does not cover a sufficient passing band depending on the type of utilisation.
A second solution consists in avoiding resisting the external pressure, the latter being opposed directly by pressure inside the shell, without a complicated system, as in patent applications FR 2 671 928 and 2 674 927 issued on Jul. 24, 1992 and filed by the French State, General Delegation of Armament, and called "Directive Electro-acoustic transducer". The device comprises a shell with cylindrical walls and bottom separated by slots obturated by a ductile diaphragm, the shell being closed by a flexible diaphragm which delimits a cavity filled with oil. This allows a transmission and pressure balance inside, but also due to the transparency of the flexible diaphragm to acoustic waves, the retransmission of the rear waves in the ambient medium, which creates a resonance peak of high frequencies, with a drop of emission level between the latter and that of the basic frequencies, reducing the power, and therefore the total emission area swept.
A third category of solution makes it possible to solve the mechanical and/or pneumatic problems of the first solution and corresponding frequency narrow band, as well as the problems related to the emission level drops and shift towards the high frequencies of the second solution. This category is described in patent application FR 2 665 998 of May 5, 1988 filed by the French State, General Delegation of Armament. It consists of using a rigid (but not airtight) shell, making it possible to delimit a cavity filled with the ambient fluid at the rear part of the pavilions, wherein closed, airtight elastic tubes filled with gas are placed such that the Helmholtz resonance frequency is close to the fundamental frequency of the axial vibrations of the vibrating assembly. A wide good range of emission frequencies is obtained thanks to 2 peaks corresponding to the own frequencies, one being linked to the transducer mechanical vibrations, and the other to the cavity, and with a maximum attenuation of 5 dB between two peaks.
Furthermore, the problem has been reported with the resistance to immersion pressure of the external shell of the elastic tubes, where their smaller diameters make it possible to obtain a less heavy assembly. But for great depths, it is compulsory to increase the tube resistance, thus limiting their elasticity and thus it is impossible to obtain very low frequency emitters, and the transducer assembly is loaded.
Finally, a fourth category of solutions make it possible to avoid limiting the depth while keeping a wide and low enough frequency band without complications in their execution. This category is developed with a shell made of a material resisting the elastic pressure, and comprising an opening, whose dimensions are determined in order that by coupling the shell elasticity to the mass of fluid located in this opening, the Helmholtz frequency of the shell cavity is close to the fundamental frequency of the vibrations of the transducer assembly. This category of solution is mainly developed for transducers such as those described in the introduction of the present description, comprising a cylindrical, rigid, hollow shell, open at both axial extremities, and inside which two identical electro-acoustic motors are installed coaxial to the latter, on both sides of a central countermass, and whose opposite extremities are surrounded by a pavilion.
However, if such a solution allows a good shift of the emission range towards the low frequencies compared to the second category of previous solutions, while keeping a wide enough range of frequencies between the obtained two peaks of emission resonance, an attenuation of more than 10 dB between both of them is noticed, and this is penalising to cover the desired ranges of emission with a sufficient power level throughout this range width.
The problem therefore consists in being able to carry out low frequency acoustic transducers in a fluid, without any depth limitation, without loading or increasing the volume and/or the complexity of these transducers, and with a wide enough band width of emission frequencies, without significant drop of level attenuation all along this band width.